Why is one inline-block element slightly higher than others

First off here is the code. I'm creating a math equation game where you are supposed to guess the operator that is missing from the equation. The operators are images that are inserted with javascript, and when one of the operator images is inserted into a div with a class of math-box, the div is raised slightly higher than all the other div tags. I'm trying to figure out why that is, I think I have a solution to make all the "math-box" classes around the same height level and that is through floating. But, I do want to want to know why is it elevating higher than all the others